6. Calculate the change in internal energy for each of the following systems:

a. 100 joules of energy is added to a system that does 40 joules of external work.
b. 75 joules of energy is removed from a system that has 50 joules of work done on it.
c. 150 joules of energy is added to a system that has 75 joules of work done on it.

7. Explain why a body of water freezes from the top down rather than from the bottom up.